Welcome to FDMB!!
You are definitely on the right track with feeding all of your cats the same food. However, the DM kibble is way too high in carbohydrates for a diabetic cat. (I know you're thinking that the vet said this is diabetic food. Read the label carefully. It says, "dietetic" not diabetic.) The canned DM is acceptable with regard to carbs but many cats get tired of it because the ingredients are suboptimal. This is a link to a website on
feline nutrition that is authored by a vet. There is a great deal of information on the site. The author, Lisa Pierson, DVM is not a fan of giving any cat dry food. There are a couple of reasons. Cats are obligate carnivores and there is a good deal of unnecessary ingredients in dry food (my favorite is powdered cellulose -- also known as sawdust) including fruits and veggies. The other reason is that cats have a limited thirst drive and dry food is, well, dry and doesn't have enough moisture. This is a
chart from her website that lists out most of the canned foods available in the US along with the amount of carbohydrates. We consider low carb to be under 10% although most members feed their cat in the 3 - 7% range and many feed either Friskies or Fancy Feast pate style foods. You have lots of choices.
If you've started insulin, we strongly encourage people to start home testing. Again, your vet may not have mentioned this. The best way to keep your cat safe is to test his blood glucose several times during the day. This is what humans do. Without home testing, you have no way to know if Hossy is in low blood glucose numbers and you're then giving an insulin injection which could cause your cat to have a symptomatic hypoglycemic episode. This is
information on home testing. Most members use a human glucometer. The Walmart Relion brand is inexpensive and the strips are also inexpensive.
Could you let us know what insulin you're using? There are guidelines from the American Animal Hospital Assn regarding which insulins are recommended for the the treatment of feline diabetes.
I don't want to overwhelm you with too much information. The front end of this learning curve is huge. Please let us know how we can help.